1. Field of the Invention
The present invention relates to optical communications through the use of optical signals which are transmitted along optical fibers and, more particularly, to a system and method for minimizing the effects of chromatic dispersion and self-phase modulation (SPM) during the transmission of such signals. The invention is especially suitable for use in data communications over single-mode optical fibers by pulses which can represent, as by pulse code modulation (PCM), bytes of data.
2. Description of the Prior Art
Optical fiber systems have the potential for achieving extremely high communication rates. Existing single mode fiber systems have already demonstrated transmission rates in the multi-Gbits/second range. Although quite large, however, this represents only a small fraction of the available bandwidth. Full utilization of the low-loss window in the 1.3-1.5 micrometer region represents a potential available bandwidth on the order of 30 THz. Thus, future fiber-optic systems will likely be capable of operating at data rates approaching and even exceeding 50 Gb/s. At these extremely high data rates the modulation bandwidth is so large that even for an ideal source without chirp or phase noise, fiber dispersion broadens the optical pulse and thus limits transmission.
The majority of currently installed single mode fiber is dispersion optimized for 1310 nm wavelength and is typically referred to as "standard" SM fiber. Once fiber losses are compensated for by Erbium-doped optical amplifiers, the ultimate speed and power limitations on data transmission rates in the standard fiber result from chromatic dispersion and non-linear optical effects such as self-phase modulation (SPM). Applications of 1550 nm transmission systems on standard SM fibers have therefore attracted considerable attention to compensation schemes for chromatic dispersion and SPM.
It has, for example, been proposed to accomplish chromatic dispersion compensation in systems employing directly modulated lasers by measuring the dispersion characteristic of each section of standard SM fiber between amplification/compensation sites or nodes of the optical path and to then insert an appropriate amount of dispersion compensating fiber (DCF) at the end of each section. For one example of this technique, reference may be had to U.S. Pat. No. 5,218,662, entitled FIBER OPTIC CABLE SYSTEM AND METHOD FOR DISPERSION COMPENSATION AT NODES BETWEEN END POINTS, issued to Dugan on Jun. 8, 1993. The strong negative dispersion coefficient of the DCF fiber is utilized to offset the positive dispersion coefficient of the standard fiber. However, this concept does not consider or address the existence of non-linear effects in the fibers. In most applications, high optical power is launched into standard fiber to transmit the signal over long distances. This high power inevitably causes nonlinear effects in the fiber; the effects, particularly SPM, induce spectral broadening of the optical signal and therefore distort the optical pulse. This SPM effect must be compensated in order to achieve long distance transmission. Moreover, the method proposed by Dugan employs a directly modulatable laser transmitter, which inherently produces broad spectral signals. Therefore, this method requires relatively accurate compensation of the dispersion.
Nobuo Suzuki et al., in a paper entitled "Simultaneous Compensation of Laser Chirp, Kerr Effect, and Dispersion in 10 Gb/s Long Haul Transmission Systems", 11 Journal of Lightwave Technology, No. 9 (September 1990), discuss a dispersion compensation technique in which laser transient chirp and SPM are simultaneously compensated by equalizing fibers inserted within certain intervals before the EDFA's. Like the Dugan system, however, the Suzuki technique is limited to systems in which a directly modulated laser operating in the SLMQW structure is utilized. This technique is therefore inapplicable to externally modulated transmission systems in which a laser operated in CW mode is employed, and no transient chirp is available to compensate the SPM.
Consequently, it would be advantageous to provide a 1550 nm optical fiber communication system employing embedded standard single mode fibers in which chromatic dispersion and self phase modulation are compensated only by the optical repeaters newly installed. That is, the simultaneous compensation in such a system would be achieved without changing any parameters of the existing transmission systems.